Robert_Charlton - 10:27 am on May 6, 2012 (gmt 0)
Google Search History is on by default. As I mentioned in this discussion from July, 2011, to turn search history off, you paradoxically need to be logged in, not logged out....
Google Personalization even when Logged out?
Also, though, note that one of the Google's announced search quality changes for April [webmasterworld.com] is...
Better query interpretation. This launch helps us better interpret the likely intention of your search query as suggested by your last few searches.
I'm wondering whether this month's "improvement" is a part of search history, or independent of it... perhaps using cookies or Google serp url modifications. Does it rely on search history, or work even if search history is turned off?